Systems which collect optical images of a given substrate can be divided into two categories depending upon the method by which they obtain an image of a given area: area imaging systems and scanning systems. In the former, a whole area of the substrate is illuminated at once and imaging optics are used to project an image of that area or a part of it upon a detector array, typically a CCD camera. Classical optical microscopy works in such a fashion. In another version of imaging systems, a spot, rather than an area, is illuminated and scanned upon the substrate, and the transmitted or reflected light is measured by one or more detectors either directly or after passing through collection optics. In those cases, the illuminating beam may be scanned across the surface in both directions or in just one direction with mechanical motion of the substrate with respect to the beam used to obtain the two dimensional area image.
FIG. 1 exemplifies an area imaging system. It includes a light source 10 which illuminates the object to be inspected (in FIG. 1 the illumination is done via a beam splitter 12 and objective lens 14). The objective lens 14 collects the light reflected from the object and provides an image of the inspected object upon the detector array 16. In contrast, FIG. 2. exemplifies a scanning system. In FIG. 2, a light source 20 generates a light beam 26 which is scanned across the inspected area, 18, using a scanning mechanism 22. The scanning mechanism provides linear scanning, 28, in one direction, while the object is mechanically moved in the perpendicular direction, thereby scanning a two-dimensional area of the object 18. Detectors 24A and 24B are used to detect light reflected from the object.
The resolution characteristics of imaging systems is defined by the point spread function - the form in which a system will image a mathematically ideal spot. An explanation of this phenomenon can be found in Joseph W. Goodman, Introduction to Fourier Optics, 2nd edition, McGraw-Hill, 1996, chapter 5. For ideal, diffraction limited, collection optics and a sufficiently dense detector array the system's point spread function is set by the collection optics, typically by the objective lens 14. Its width is proportional to the wavelength and inversely proportional to the imaging optics' numerical aperture.
Unlike the system's point spread function, the system's modulation transfer function (MTF), which is a measure of its spatial frequency response, depends on the illumination optics as well. (See, Joseph W. Goodman, Introduction to Fourier Optics, 2nd edition, McGraw-Hill, 1996, chapter 6.) When the illumination across the surface has a definite phase relationship, the system is termed perfectly coherent. When the illumination is incident from multiple directions, the definite phase relationship is lost and the system becomes either partially coherent or incoherent. When the sine of the illumination angle or its numerical aperture is equal to the imaging numerical aperture or greater, the system is termed totally incoherent. The cutoff frequency of the system's MTF is defined as the spatial frequency above which the system has zero response. For coherent systems, the MTF cutoff is half that of incoherent ones. Furthermore, when sharp features such as edges are imaged by coherent systems, the images display oscillations termed "ringing".
In scanning systems, on the other hand, light is focused upon a small spot of the substrate to be imaged and is moved across it in one or two dimensions. Some of the light that is reflected from the spot is collected upon at least one detector which is sequentially sampled. The detector's output along with the knowledge of the spot's location at any given time is used to reconstruct an image of the area scanned. The detector may capture any light scattered into a given spatial angle or may have limiting apertures which limit incoming light to that reflected from the focal spot of the scanning illumination, such as in confocal laser microscopy.
A key disadvantage of scanning systems is their serial, rather than parallel nature, i.e., it takes longer to construct an image using a scanning system. According to the Nyquist theorem, sampling the detectors at a rate that is more than twice the cutoff frequency does not increase the system's frequency response. Therefore meaningful pixels can only be generated at a rate proportional to the time in which the scanning spot can transverse itself. This rate is set by the scanning mechanism (typically an acoustic-optical deflector (AOD), a rotating polygon, or resonant mirrors) and is typically limited to several tens of Mega-pixels per second. Area imaging systems do not suffer from this limitation as a large area could be simultaneously imaged upon a detector array and read out in parallel through more than one output or tap.
A key advantage of scanning systems over area imaging systems is the ability to use laser sources which have both a high brightness and a potentially narrow spectral emission range. The latter is particularly important for UV optical systems where it is difficult to correct for spectral aberrations, When lasers are used in area imaging systems, they lead to half the MTF cutoff frequency of incoherent illumination as well as coherent phenomena such as ringing of edges and speckles. Schemes exist for destroying the coherence of the laser sources but they inevitably add to the system's complexity as well as lead to a loss of optical power.
Besides the division between area illumination-based systems and laser-spot scanning-based systems, imaging systems are also divided by the direction of the illumination with respect to the collection optics. If the illumination impinges upon the substrate from a direction such that the specularly transmitted or reflected light is collected by the imaging optics, the system is termed "bright field"(BF). The imaging system depicted in FIG. 1 is a bright field system, i.e., since the wafer 18 is illuminated perpendicularly, it reflects the light also perpendicularly--in the direction of the detector array 16. If, on the other hand, the illumination arrived from a direction which is outside the collection angle of the imaging optics, the system is termed "dark field"(DF). The scanning system depicted in FIG. 2 is a dark field system, i.e., the light beam 26 impinges upon the wafer perpendicularly and hence the light would normally be reflected perpendicularly--not in the direction of detectors 24A and 24B. Only light reflected from irregularities on the wafer 18 will be reflected towards detectors 24A and 24B. Dark field imaging is used to enhance edge phenomena by collecting only the diffusively reflected light. When used for optical inspection, dark-field laser scanning systems (such as the Orbot WF-731 available from Applied Materials of Santa Clara, Calif.) greatly improve the signal to noise ratio for small, three dimensional objects in a mostly flat background. Furthermore, using several detectors located in different angles within the dark field (such as in the WF-731) increases the chance of defect capture and reduces the chance for false alarms. As a result, it is possible to detect defects that are significantly smaller than the system's spatial resolution and hence it is possible to scan the substrate using relatively large pixels and thus achieve a very high system throughput.
Other inspection systems, such as the one described in U.S. Pat. No. 5,805,278 by J. Danko and the KLA-2230, available from KLA of San Jose, Calif., use dark-field illumination and area imaging optics to enjoy some of the same advantages. Yet other systems, such as the Orbot WF-736, also available from Applied Materials, use laser scanning and a combination of dark-field and bright field detection. A limitation on such systems is that the resolution of the bright-field channel is equal to that of the dark-field ones although the signal to noise ratio for the bright-field channel is inherently inferior. It therefore may be difficult to detect defects that are small and flat.
Another system of interest is described in PCT application WO 96/39619. That system includes an area imaging system which utilizes two light sources of different wavelengths to simultaneously collect both a dark-field and a bright field image. This system has the disadvantage of requiring what are essentially two separate imaging paths including two illumination sources, two detector arrays and alignment between these two channels.